Clean water is important for many industrial processes and for our daily life. The petroleum industry generates large amounts of wastewaters with high concentrations of oil, including produced water brought to the surface during oil drilling and gas production and refinery wastewater. Produced water accounts for the largest portion of wastewaters in petroleum industry and contains a wide range of contaminants, including salts, heavy metals, oil, suspended solid particles, dissolved organics, and small amount of chemical additives used for drilling, and its composition varies from well to well and from time to time. Depending on its use, produced water needs to be treated at different levels to ensure its reuse and recycling within oil and gas drilling operations, beneficial reuse outside of operations, and surface discharge. However, no matter for onshore disposal or reuse as process water or for off-shore discharge into the sea, essentially almost all oil and grease contaminants in produced water must be removed. Refinery wastewater, which constitutes another large stream of wastewaters, contains hydrocarbons even after conventional wastewater treatment due to its limited biological degradation, and thus also needs further treatment to remove remaining hydrocarbons for discharge or reuse. Hydrocyclones and dissolved air flotation have been used to quickly and effectively remove a large portion of the free oil droplets, but the quality of thus treated water is not high enough for discharge or reuse. Membrane filtration is a highly promising technology to further treat the resultant water with low concentration of oil to obtain almost oil-free water.
In particular, ion removal from water is important in water purification, such as desalination, produced water treatment in shale gas production, nuclear wastewater treatment, etc. Ion separation using membrane technology can greatly reduce energy cost in industrial processes. However, currently there are no technologies available to fabricate ultrathin (e.g., less than 5 nm, much less sub-nanometer), graphene-based membranes that can highly selectively remove ions from water.